This proposal describes the discovery and development of low molecular weight gp41 inhibitors effective against Human Immunodeficiency Virus (HIV-1) fusion. Fusion inhibitors possess excellent characteristics for interrupting HIV transmission and preventing disease progression, since they block initial infection of healthy cells and cell-to-cell spread of infection. Yet there are currently no small molecule inhibitors of fusion, and peptide fusion inhibitors possess some undesirable characteristics such as high cost and susceptibility to proteolysis. The objective of this application is to develop small molecule lead compounds which bind to gp41 with high affinity, preventing the association of N- and C-helical domains that is a central feature of the fusion reaction. The proposal implements a systematic structure-based approach to fusion inhibitor development in three Specific Aims: (1) Modular subsections of the gp41 N-helical coiled coil focused on or around a known hydrophobic pocket will be examined for small molecule binding, through the development of fluorescence, NMR and biological assays to detect and quantitate binding. (2) Novel NMR methods for obtaining explicit structural constraints defining bound ligand conformation and orientation will be developed, providing quantitative structure-activity relationship data for computer modeling and synthesis of improved inhibitors. (3) Compounds with peptidomimetic scaffolds have shown promising binding characteristics in the hydrophobic pocket and will form the basis of design strategies for preparing novel inhibitors, using binding, structural and biological assays for evaluation and optimization. Additional or adjacent hits and structural data obtained during assay application will be incorporated into the lead optimization process. Cell-cell fusion and viral infectivity assays will be applied to provide corroboration of binding studies and to ensure that lead candidates have the desired in vitro properties. This approach will allow testing of the central hypothesis that small molecules binding to the hydrophobic pocket and potentially extending into grooves adjacent to the hydrophobic pocket can be developed into highly potent fusion inhibitors, rivaling peptides. The studies described will provide efficient methods to detect new generations of inhibitors and obtain structural information on their complexes with gp41. Thus they will contribute to a gap in knowledge of the mechanism of gp41 inhibition, since there are currently no experimental data describing the details of small-molecule - gp41 complexes. The significance of this proposal lies in its potential to contribute specifically to HIV-1 fusion inhibitor development and also to the development of techniques applicable to a wide range of Class 1 viruses which use a similar fusion mechanism to HIV-1, a broader goal relevant to NIH's mission of enhancing fundamental knowledge to treat human disease. The long-term goal of the project is to generate drug-like compounds active against HIV-1 fusion, suitable for therapeutic or microbicide use, which could be used to counter or prevent the millions of new infections that occur annually worldwide. PUBLIC HEALTH RELEVANCE: The prevalence of HIV infection remains a significant public health problem due to the development of viral strains resistant to current treatments, and due to spread of the virus from individuals who are unaware they are infected. This proposal seeks to apply structure-aided drug design to develop low molecular weight HIV fusion inhibitors which prevent viral entry and the cell-to-cell spread that causes immune deficiency. Drugs in this class would be useful in microbicides to prevent infection, or could be given orally to treat infection as an alternative to the currently used fusion inhibitor Enfuvirtide(R), which must be given intravenously.